A synchronous motor is supplied with a three-phase alternating current from an inverter and rotates the rotor by causing the stator to generate magnetic fields by letting the current flow to coils mounted on the stator. A driving system of the synchronous motor flexibly controls the driving of the rotor by controlling the current applied to the coils appropriately in accordance with positions of magnetic poles of the rotor.
However, torque generated by the synchronous motor is not kept constant and ripples periodically. This is due to the structure of magnetic circuits mainly determined by shapes of the stator and the rotor, as well as a current waveform of the current applied to the coils. The torque ripple is composed of ripple components each having a different period. Generally speaking, a ripple component having a period of 60 electrical degrees is prominent. It is known that the periodicity of torque components varies according to shapes of the stator and the rotor, namely, a combination of the number of slots of the stator and the number of magnetic poles of the rotor. It is also known that the torque ripples emerge as a result of the current waveform deviating from a sinusoidal pattern.
Meanwhile, a recent development of power electronics technologies has resulted in broader use of synchronous motors. It is becoming popular to adopt synchronous motors driven by an alternating current to a vehicle motor using DC power supply. The use of synchronous motors in vehicles, such as a hybrid electrical vehicle and an electrical vehicle, strongly requires a further progress of output power and performance (i.e. reduced vibration and noise) and cost reduction of a motor drive system. Further, in the use in vehicles, along with high reliability, a failsafe mechanism is required so as to prevent catastrophic results due to a possible partial malfunction of the synchronous motor system.
For example, Patent Document 1 discloses a motor drive system for increasing voltage applied to an inverter and a motor by providing a boost circuit between a DC power supply and the inverter. Since an output power of a motor increases in proportion to the applied voltage, the increase in the voltage through a boost operation of the boost circuit leads to an increase in the output power of the motor.